Recently, DVDs (Digital Versatile Discs) have been widely used as optical information media (hereinafter, referred to as “optical discs”). As DVDs, there exist DVD-ROM discs for reproduction only, DVD-RW discs which are rewritable, and DVD-R discs which are of the write-once type.
A DVD-RW disc has a memory capacity of 4.7 GB, with which at least 2 hours of NTSC moving pictures are recordable. The DVD-RW disc includes an inner lead-in area and a data area which is provided to outside the lead-in area in which information such as moving pictures or the like is to be recorded. The lead-in area has concave and convex type pits formed spirally, the pits representing information such as, for example, the book type, version of the Standards, disc size, and disc structure. The data has a guide groove which wobbles minutely formed therein.
A DVD-RW disc is produced as follows. First, a master disc including a lead-in area having pits and a data area having a guide groove is produced. From the master disc, a stamper is produced. Minute features of a surface of the stamper are transferred to a plate of resin, thus forming a first resin substrate having a thickness of 0.6 mm. Then, a recording material layer containing a phase-change material is formed on a surface of the first resin substrate by sputtering or the like. A second resin substrate having a thickness of 0.6 mm is placed on the recording material layer. Thus, a DVD-RW disc having a thickness of 1.2 mm is produced. In the DVD-RW disc, the recording density of the information in the lead-in area is equal to the recording density of the information in the data area. A light beam used for recording information on the DVD-RW disc is directed to the recording material layer through the first resin substrate which is a replica, of the master disc, produced using the stamper.
The information represented by the pits formed in the lead-in area is the same among an identical type of optical discs. Therefore, in order to reduce the cost of mass-production of the optical discs, the pits in the lead-in area are preferably reproduced using a stamper by an injection technique or the like.
In general, a signal obtained by the concave and convex type pits has a progressively smaller amplitude for pits progressively shallower than λ/4. Thus, the signal-to-noise ratio (S/N ratio) of such a signal is lowered. By contrast, a signal obtained by a recording mark formed in the recording material layer on the guide groove has a progressively smaller amplitude for a progressively deeper guide groove due to the amount of light reflected by the guide groove being decreased. Thus, the S/N ratio of such a signal is lowered. In order to solve this problem, a spiral guide groove is formed to be as shallow as λ/10 to λ/16 (λ: wavelength of light) and the pits are formed to be deeper than the guide groove. Therefore, while the master disc of the DVD-RW disc is produced, the amount of light is increased for forming the pits so that the depth of the pits is substantially equal to the thickness of a resist layer, and the amount of light is decreased for forming the guide groove so that the depth of the guide groove is smaller than the thickness of the resist layer.
BS digital broadcasting has started, and thus contents having high definition images are now broadcast. Both the memory capacity and the recording speed of conventional optical discs are insufficient to accumulate such high definition images. There is a strong demand for a new optical disc having a memory capacity of as large as 25 GB and a recording speed of as high as 33 Mbps. In order to meet this demand, an optical disc for higher density recording has been developed using purple laser light having a wavelength of as short as 405 nm and an objective lens having a numerical aperture of as high as 0.85. Realization of such high density recording requires further reduction in the noise caused by the guide groove so that a signal obtained from the recording mark has a satisfactory S/N ratio.
The above-described DVD-RW disc has the following problem in fulfilling such a requirement. Since the depth of the guide groove and the depth of the pits are different from each other, it is necessary to stop a photolithography step of the resist layer when the guide groove is formed to a depth into the resist layer in the production of a master disc. Therefore, it is difficult to form the groove to have a uniform depth in the entirety thereof, and thus it is difficult to produce a master disc having a guide groove which provides a signal having a low level of noise. The yield of the master discs is reduced; namely, cutting needs to be repeated a plurality of times before obtaining one satisfactory master disc. This results in an increase in the cost of the optical discs as final products.